Ignore untracked files when checking for dirty working copy

The working will not be considered dirty if no tracked files have been modified, added or deleted
Check for dirty working copy can optionally ignore untracked files

The default is that untracked files are considered dirty. To ignore pass ignore_untracked_files=True in the call to git_seed().

Moved the logic to check for dirty working copy to a little helper function: _is_dirty()
better _is_dirty_() check
Merge pull request #2 from the-gigi/master

Ignore untracked files when checking for dirty working copy